Direct conversion receiver

Abstract

A direct conversion receiver for a radio system has an antenna and blocking filter connected to an amplifier. The input signal is split and mixed with an in-phase signal and a quadrature phase signal generated by an oscillator in a mixer circuit. An output from each mixer circuit is applied to a low pass filter and to an input of a limiting circuit. The output from each low pass filter is applied to respective circuits, a first of which is arranged to sum the in-phase and quadrature phase signals, and the second of which is arranged to subtract the in-phase and quadrature phase signals to generate a respective output signal having an axis intermediate the in-phase and quadrature phase signal axes. These signals, together with the in-phase and quadrature phase signals are passed through a limiting circuit respectively, to a decoder circuit for recovering the data. The output of the limiting circuits represent signals quantized to eight possible phase states separated by 45°. In an alternative embodiment a ratiometric combiner may be used instead of the summation circuits, particularly for π/4-DQPSK modulation, where the phase excursion per symbol is ±45° or ±135° depending upon the bit pair combination. In the latter embodiment a minimum of eight axes are generated giving sixteen sectors and 22.5° phase resolution.

I claim: 
     
       1. A direct conversion receiver comprising: means for receiving an input signal transmitted across a radio medium and represented by a vector;   means for generating in-phase and quadrature phase signals from the received input signal;   means for generating in-phase and quadrature phase output signals in the form of first bursts of hard-limited signals;   circuit means arranged to generate additional axes intermediate to axes of the in-phase and quadrature phase signals, from which additional axes second bursts of hard-limited signals are generated; and   a decoder circuit arranged to receive the first and second bursts of hard-limited signals and to generate data corresponding to the received input signal; wherein the decoder circuit comprises: a combination logic circuit arranged to receive said first and second bursts of hard limited signals, and to give a sector output value from zero to seven, when four axes are used, representing a phase sector in which the received input signal instantaneously resides; and   means for subtracting a previous sector output value from a present sector output value to generate a series of pulses dependent upon a direction of rotation of the vector.     
     
     
       2. A receiver as claimed in claim 1, wherein the series of pulses are applied to an averaging circuit and to a limiting circuit arranged to extract the data from said series of pulses. 
     
     
       3. A direct conversion receiver comprising: means for receiving an input signal transmitted across a radio medium and represented by a vector;   means for generating in-phase and quadrature phase signals from the received input signal;   means for generating in-phase and quadrature phase output signals in the form of first bursts of hard-limited signals;   circuit means arranged to generate additional axes intermediate to axes of the in-phase and quadrature phase signals, from which additional axes second bursts of hard-limited signals are generated; and   a decoder circuit arranged to receive the first and second bursts of hard-limited signals and to generate data corresponding to the received input signal; wherein the circuit means comprises a ratiometric combiner arranged to generate eight axes intermediate of the in-phase and quadrature phase signal axes;   output signals from the ratiometric combiner, and the in-phase and quadrature phase signals are applied to respective limiting circuits, output signals from which are applied to an input of the decoder circuit respectively; and   the decoder circuit includes:     input logic means arranged to receive the output signals from the respective limiting circuits and arranged to generate a sector output signal in the range of 0 to 15, when eight axes are used; and   means for subtracting a previous sector output signal from a present sector output signal in order to generate a series of output data pulses.   
     
     
       4. A receiver as claimed in claim 3, wherein the series of output signals are applied to an averaging circuit, the output of which is applied to an input of three decision circuits respectively, and arranged to detect a magnitude and polarity of phase shift, an output of each decision circuit being connected to an output logic circuit arranged to generate data output signals on output data lines.